FIG. 1 is an exploded perspective view showing a backlight type surface light source device 11 according to a first example of the related art and FIG. 2 is a schematic sectional view of the same. The surface light source device 11 includes a light guide 12 for confining light, a light-emitting portion 13, and a reflector 14. The light guide 12 is formed from a resin having a great refractive index such as polycarbonate resin or polymethylmethacrylate, and a pattern 15 is formed on a bottom surface of the light guide 12 by processing the surface to have concavities and convexities having a substantially semi-circular sectional shape or printing dots of a diffuse reflection ink on the same. The light-emitting portion 13 is provided by mounting a plurality of so-called point light sources 17 such as light-emitting diodes (LED) on a circuit board 16, and this portion faces a side surface of the light guide 12 (a light-entrance surface 18). The reflector 14 is formed from a material having a high reflectivity, e.g., a white resin sheet, and two side parts of the same are attached to a bottom surface of the light guide 12 using double-sided tapes 19.
As shown in FIG. 2, light emitted from the light-emitting portion 13 and guided into the light guide 12 through the light entrance surface 18 (the light is indicated by an arrow) is guided in the light guide 12 by being repeatedly subjected to total reflection between a light exit surface 20 of the light guide 12 and a surface opposite to the same. The light guided in the light guide 12 undergoes diffuse reflection by impinging on the pattern 15. Beams of the light reflected to impinge on the light exit surface 20 at an angle smaller than the critical angle of the total reflection are output from the light exit surface 20. Beams of the light which have leaked out through locations of the bottom surface of the light guide 12 where the pattern 15 does not exist are reflected by the reflector 14 back into the light guide 12, which prevents any loss of luminous energy at the bottom surface of the light guide 12.
As shown in FIG. 2, the light exiting the light exit surface 20 of the light guide 12 as thus described exits in a direction substantially parallel to the light exit surface 20 of the light guide 12. Thus, the light exit surface 20 has low luminance in a direction perpendicular to the light exit surface 20 (hereinafter the direction may be expressed as “frontal”) and is therefore very dark when viewed frontally unless some measure is taken. For this reason, a diffusing sheet 21 having a random pattern is disposed opposite to the light exit surface 20 of the light guide 12. The light exiting the light exit surface 20 substantially parallel to the same is scattered by the diffusing sheet 21, and the quantity of light exiting the direction perpendicular to the light exit surface 20 is thereby increased. In the surface light source device 11 of this type, however, light transmitted by the diffusing sheet 21 resembles Lambert light in that it has very wide directivity because the light is randomly diffused by the diffusing sheet 21. Thus, the surface light source device 11 has had low frontal luminance.
Under the circumstance, various ideas for improving the frontal luminance of a surface light source device have been proposed. FIG. 3 is an exploded perspective view showing a backlight type surface light source device 31 according to a second example of the related art, and FIG. 4 is a schematic sectional view of the same. In the surface light source device 31, a plurality of microscopic deflecting pattern elements 35 having a substantially triangular sectional shape are provided on a bottom surface of a light guide 32. In order to achieve a uniform luminance distribution, the pattern density of the deflecting pattern elements 35 is low in the neighborhood of a light-emitting portion 33, and the pattern density gradually increases with the distance from the light-emitting portion 33. A mirror reflection sheet 34 is disposed opposite to the bottom surface of the light guide 32. A bar-shaped light emitting portion 33 constituted by a cold-cathode tube is disposed in a position opposite to a light entrance surface 38 of the light guide 32. A prism sheet 42 is disposed in a position opposite to a light exit surface 40 of the light guide 32, and an uneven diffusing board 41 is placed on the same. On the prism sheet 42, prisms 42a having a triangular sectional shape linearly extending in a direction parallel to the longitudinal direction of the light-emitting portion 33 are arranged parallel to each other at a constant pitch.
In the surface light source device 31, as shown in FIG. 4, light from the light-emitting portion 33 guided into the light guide 32 is guided in the light guide 32 by being repeatedly subjected to total reflection between the light exit surface 40 of the light guide 32 and a surface opposite to the same. A light guide angle α of the light guided in the light guide 32 increases each time the light impinges on a deflecting pattern element 35. When the light is reflected to impinge on the light exit surface 40 at an angle smaller than the critical angle of the total reflection, the light exits the light exit surface 40. In this surface light source device 31 again, the light exiting the light exit surface 40 exits in a direction substantially parallel to the light exit surface 40. The light which has exited the light guide 32 in a direction substantially parallel to the light exit surface 40 is deflected in a direction substantially perpendicular to the light exit surface 40 by being transmitted by the prism sheet 42.
Let us now assume that a direction perpendicular to the light entrance surface 38 of the light guide 32 is defined as a y-axis direction; a direction parallel to the light entrance surface 38 and also parallel to the light exit surface 40 is defined as an x-axis direction; and the direction perpendicular to the light exit surface 40 is defined as a z-axis direction. It is also assumed that φx represents an azimuth angle measured in the z-x plane from the z-axis direction and that φy represents an azimuth angle measured in the y-z plane from the z-axis direction.
Light guided through the light guide 32 to exit the light exit surface 40 is not controlled in terms of directivity in the x-axis direction. Therefore, the light is widely diffused in the x-axis direction in the light guide 32, and the light transmitted by the prism sheet 42 has wide directivity in the φx direction as indicated by Δφx in FIG. 5. On the contrary, the light exits the light exit surface 40 toward a narrow range substantially in parallel with the light exit surface 40, and the light transmitted by the prism sheet 42 therefore has narrow directivity in the φy direction as indicated by Δφy in FIG. 5
A reduction in frontal luminance occurs when the directivity is too wide, whereas excessively narrow directivity degrades visibility because an image becomes less visible as a result of only a slight change in the position of the viewer's face. For this reason, in the surface light source device 31, an uneven diffusing board 41 is disposed on the prism sheet 42 as shown in FIG. 4, whereby the directivity of the light transmitted by the prism sheet 42 in the φy direction is expanded to β=20° in the y-z plane by the uneven diffusing board 41. As a result, the surface light source device 31 provides frontal luminance higher than that of the surface light source device 11 according to the first example of the related art, and it also allows any reduction in the visibility of a liquid crystal display to be prevented.
FIG. 6(a) is a plan view showing the disposition of diffusing patterns 43 on the uneven diffusing board 41. FIG. 6 (b) is an illustration showing a sectional shape of the uneven diffusing board 41 on a section thereof parallel to the z-x plane. FIG. 6 (c) is an illustration showing a sectional shape of the uneven diffusing board 41 on a section thereof parallel to the y-z plane. The diffusing patterns 43 are configured to diffuse light transmitted by the uneven diffusing patterns 41 widely in the y-z plane while limiting the diffusion in the z-x plane. Specifically, the diffusing patterns 43 formed on a surface of the uneven diffusing board 41 are uneven patterns linearly extending in a direction parallel to the x-axis and curved in a wavy shape on the section parallel to the y-z plane. Although the diffusing patterns 43 are formed to be flat and linear on the section parallel to the z-x plane, there are seams 44 in the form of concavities generated between the diffusing patterns 43 as a result of sagging at the time of molding.
Let us now assume that Λx represents the period of the diffusing patterns 43 in the x-axis direction and that Λy represents the period of the same in the y-axis direction. Then, the period Λy is smaller than the other period Λx in the plane in which light is to be diffused (that is, Λx>>Λy). The period Λx of the diffusing patterns 43 is desirably 50 to 200 μm, and the period Λy is desirably 5 to 20 μm. One reason is that the diffusing patterns 43 become visually perceivable and thereby reduce the display quality of the liquid crystal display when the periods Λx and Λy of the diffusing patterns 43 exceed respective upper limit values. Further, the patterns become difficult to fabricate, and errors generated during the fabrication of the diffusing patterns 43 become great in comparison to the shape of the patterns when the periods Λx and Λy of the diffusing patterns 43 are smaller than respective lower limit values, which can reduce the utilization of light and can result in problems such as color irregularities caused by diffracted light that becomes dominant.
FIG. 7 is an illustration explaining an effect of the diffusing patterns 43 in the y-z plane, and FIG. 8 is an illustration explaining an effect of the diffusing patterns 43 in the z-x plane. The diffusing patterns 43 have a wavy shape on the section parallel to the y-z plane. When it is assumed that γ represents the angle of a line tangential to a light exit point. Then, light exiting a diffusing pattern 43 has a diffusing angle (deflection angle) φy expressed by:φy=γ/(n−1)where n represents the refractive index of the resin from which the uneven diffusing board 41 is formed and where the refractive index of air is 1.
On the contrary, as shown in FIG. 8, light passing through the diffusing patterns 43 is not diffused in the z-x plane because the diffusing patterns 43 are flat. Diffusion occurs in the z-x plane at the seams 44 between the diffusing patterns 43. However, since the period Λx in the x-axis direction is considerably greater than the period Λy in the y-axis direction, the quantity of light diffused in the z-x plane due to the seams 44 is quite small compared to the quantity of light diffused in the y-z plane by the diffusing patterns 43. Thus, the uneven diffusing board 41 causes diffusion substantially only in the y-z plane.
FIG. 10 is a diagram showing diffusing characteristics of the uneven diffusing board 41. As apparent from the characteristic diagram, the uneven diffusing board 41 causes substantially no diffusion of light in the φx direction in the z-x plane and causes diffusion spreading at about 20° in the φy direction in the y-z plane. The directional characteristics of light exiting the prism sheet 42 are wider in the φx direction and narrower in the φy direction as shown in FIG. 9. When the uneven diffusing board 41 having diffusion characteristics as shown in FIG. 10 is placed on the prism sheet 42 having such directional characteristics, light emitted from the surface light source device 31 will have directional characteristics as shown in FIG. 11. The directivity of light which has exited the uneven diffusing board 41 is substantially equal to the directivity of light which has passed through the prism sheet 42 in the φx direction. However, the directivity of light which has exited the uneven diffusing board 41 is wider than the directivity of light which has passed through the prism sheet 42 in the φy direction. The light is diffused at about 20° on both sides of the z-axis in both of the φx and φy directions, which provides a viewing angle that is sufficient when employed in a liquid crystal display.
FIG. 12 is a perspective view showing a backlight type surface light source device 51 according to a third example of the related art, and FIG. 13 is a schematic sectional view of the same. In this surface light source device 51, a light-emitting portion 53 constituted by a point light source such as an LED is disposed in the vicinity of a corner of a light guide 52. Deflecting pattern elements 55 having a triangular sectional shape are concentrically arranged about the light-emitting portion 53 on a bottom surface of the light guide 52. The pattern density of the deflecting pattern elements 55 is small in the neighborhood of the light-emitting portion 53, and the pattern density increases with the distance from the light-emitting portion 53. A mirror reflection sheet 54 is disposed in a position opposite to the bottom surface of the light guide 52. A prism sheet 62 is disposed in a position opposite to a light exit surface 60 of the light guide 52, and an uneven diffusing board 61 is placed on the same. The prism sheet 62 is formed with prisms having a triangular sectional shape and extending in the form of arcs about the light-emitting portion 53.
A position or direction with respect to the surface light source device 51 will be expressed using cylindrical coordinates. Let us now assume that a direction perpendicular to the light exit surface 60 of the light guide 52 is defined as a z-axis direction; a radial direction about the light-emitting portion 53 is defined as an r-axis direction; θ represents an angle measured from one side of the light guide 52; and a direction perpendicular to the r-axis direction and the z-axis direction is defined as a θ-axis direction. It is also assumed that φr represents an azimuth angle measured from the z-axis in a plane parallel to the r-axis direction and the z-axis direction and that φθ represents an azimuth angle measured from the z-axis in a plane perpendicular to the r-axis direction and parallel to the z-axis direction.
In the surface light source device 51, as shown in FIG. 13, light which has entered the light guide 52 from the light-emitting portion 53 (point light source) radially spreads about the light-emitting portion 53. In addition, each of the deflecting pattern elements 55 is disposed such that the longitudinal direction of the same becomes orthogonal to the direction of connecting it with the light-emitting portion 53 (r-axis direction) in a plan view taken in the z-axis direction. Therefore, the light guided through the light guide 52 travels substantially straightly in the r-axis direction without being scattered in the θ direction even if it is reflected by the deflecting pattern elements 55. As a result, light has directivity as shown in FIG. 14 when it is substantially perpendicularly deflected by the prism sheet 62 after being reflected by the deflecting pattern elements 55 to exit the light exit surface 60 of the light guide 52. The directivity of the light transmitted by the prism sheet 62 is narrow in terms of both of directivity Δφr in the φr direction and directivity Δφθ in the φθ direction. Since the directions of beams of light guided in the light guide 52 substantially agree with the r-axis direction, the directivity Δφθ in the φθ direction is especially narrow.
Therefore, the uneven diffusing board 61 in the third example of the related art must have the function of spreading light transmitted by the prism sheet 62 in the φθ direction. For this reason, as shown in FIG. 15(a), diffusing patterns 63 extending in the r-axis direction are disposed concentrically or radially about the light-emitting portion 53. The diffusing patterns 63 are patterns that are flat as shown in FIG. 15(b) on the section indicated by P1-P1 in FIG. 15(a), and there are seams 64 between the diffusing patterns 63. The diffusing patterns 63 are patterns that are wavy as shown in FIG. 15(c) on the section indicated by P2-P2 in FIG. 15(a).
The function of the diffusing patterns 63 is similar to that of the diffusing patterns 43 in the second example of the related art (see FIGS. 7 and 8) except the direction in which light is diffused. The r-axis direction in the third example of the related art corresponds to the x-axis direction of the second example of the related art, and the θ-axis direction in the third example of the related art corresponds to the y-axis direction of the second example of the related art. Therefore, when it is assumed that Λr represents the period of the diffusing patterns 63 in the r-axis direction and that Λθ represents the period in the θ-axis direction, the period Λr in the r-axis direction is greater than the period Λθ in the θ direction (that is, Λr>>Λθ). The period Λr of the diffusing patterns 63 is desirably 50 to 200 μm, and the period Λθ is desirably 5 to 20 μm.
FIG. 17 is a diagram showing diffusing characteristics of the uneven diffusing board 61. As apparent from the characteristic diagram, the uneven diffusing board 61 causes substantially no diffusion of light in the φr direction in the z-r plane and causes diffusion spreading at about 20° in the φθ direction in the z-θ plane. The directional characteristics of light exiting the prism sheet 62 are wider in the φr direction and narrower in the φθ direction as shown in FIG. 16. When the uneven diffusing board 61 having diffusion characteristics as shown in FIG. 17 is placed on the prism sheet 62 having such directional characteristics, light emitted from the surface light source device 51 will have directional characteristics as shown in FIG. 18. The directivity of light which has exited the uneven diffusing board 61 is substantially equal to the directivity of light which has passed through the prism sheet 62 in the φr direction. However, the directivity of light which has exited the uneven diffusing board 61 is wider than the directivity of light which has passed through the prism sheet 62 in the φθ direction. The light is diffused at about 20° on both sides of the z-axis in both of the φr and φθ directions, which provides a viewing angle that is sufficient to be employed in a liquid crystal display.
However, the surface light source devices 31 and 51 utilizing the uneven diffusing boards 41 and 61 as used in the second and third examples of the related art had a problem when a liquid crystal display panel 65 was overlaid thereon to form a liquid crystal display in that moiré fringes M having a period of several mm were generated on the surface of a liquid crystal display panel 65 to degrade the display quality of the liquid crystal display. FIG. 20 shows how moiré fringes are generated in practice. FIG. 21 is a part of the liquid crystal display panel 65 in which one picture element was formed by a red pixel (R), a green pixel (G), and a blue pixel (B) and in which picture elements had an aperture period Λc=120 μm. When a liquid crystal display panel 65 as shown in FIG. 21 is overlaid on a surface light source device 31 according to the second example of the related art utilizing an uneven diffusing board 41 having a period Λx=200 μm in the x-axis direction and a period Λy=20 μm in the y-axis direction, moiré fringes as shown in FIG. 20 appear in a cycle of several mm to degrade an image on the liquid crystal display. In the case of the surface light source device according to the third example of the related art, moiré fringes not shown in a radial pattern are generated.
It is supposed that moiré fringes are generated on the surface of a liquid crystal display panel for the following reason when an uneven diffusing board having linear diffusing patterns arranged thereon. FIG. 22 shows the state of the uneven diffusing board 41 observed using a microscope, the board being illuminated from the back side thereof (FIG. 50 shows a microphotograph of the same). Luminance is higher at the seams 44 between the diffusing patterns 43 because there is no diffusion of light in the φy direction as seen at the diffusing patterns 43, and the uneven diffusing board 41 therefore has peaks of luminance along the seams 44. Since the period of the seams 44 is about 100 μm, the peaks of luminance of the uneven diffusing board 41 are repeatedly generated also in that cycle. The period of luminance peaks and the period Λc of the picture elements of the liquid crystal display panel 65 are similar values, and it is therefore supposed that the moiré fringes M are generated as a result of interference between them.
The use of the uneven diffusing boards 41 and 61 as used in the second and third examples of the related art resulted in another problem in that color flickers as shown in FIG. 23 were generated on the surface of the liquid crystal display panel 65. The reason is supposed as follows. For example, let us assume that the diffusing patterns 43 provided on the uneven diffusing board 41 has a period Λy=10 μm in the y-axis direction and that each of the red, green, and blue pixels of the liquid crystal display panel 65 has an aperture width W=35 μm. When such surface light source device and liquid crystal display panel are combined, as shown in FIG. 24, the number of diffusing patterns 43 extending through each pixel varies between two and three on a case-by-case basis. For example, in the case shown in FIG. 24, three diffusing patterns 43 extend through the aperture of a red pixel (R) and the aperture of a blue pixel (B), whereas only two diffusing patterns 43 extend through the aperture of a green pixel (G). Therefore, there is a maximum difference of 1.5 times between emission intensities of the pixels. In addition, since pixels in different emission colors have the higher intensity depending on their locations, the emission color of pixel having the higher intensity depends on locations, coloring occurs in different colors depending on locations, which is supposed to appear as color flickers.
A board having granular uneven patterns randomly formed thereon may be used as the uneven diffusing board to be used in the second or third example of the related art. FIGS. 25(a), 25(b), 26(a), and 26(b) are illustrations for explaining an uneven diffusing board 71 having different patterns. As shown in FIG. 25(a), the uneven diffusing board 71 has cyclic patterns 72 which are periodically arranged from left to right and from top to bottom such that substantially no gap is left between them. As shown in FIG. 10(b), the cyclic patterns 72 are provided by arranging concavities 73 at random with substantially no gap left between them. Widths H and D of one cyclic pattern 72 in the vertical and horizontal directions are made greater than the size of a pixel of the liquid crystal display panel in order to prevent moiré fringes, and they are both preferably in the range between 100 μm and 1 mm, inclusive. The concavities 73 constituting the cyclic patterns 72 are uneven in dimensions, and they desirably have an outer diameter G in the range between 5 μm and 30 μm, inclusive (in particular, a diameter of about 10 μm is preferable). The concavities 73 are concave-lens-shaped as shown in FIGS. 26(a) and 26(b).
Since the uneven diffusing board 71 has extraordinary diffusing characteristics, the uneven shape of the patterns must be accurately controlled. In doing so, the uneven patterns can be accurately formed by periodically arranging one uneven pattern because all uneven patterns will thus have the same shape and can be fabricated similarly. According to such a method, however, moiré fringes are more likely to be generated on the screen of the liquid crystal display, and pixels are more likely to become perceivable. On the contrary, when it is attempted to dispose uneven patterns at random, the shape and size of the uneven patterns must be varied one by one, which makes it difficult to fabricate them in accurate shapes. Further, the characteristics of the uneven diffusing board can vary depending on locations. For this reason, on the uneven diffusing board 71, the cyclic patterns 72 are formed by disposing concavities 73 having random shapes and dimensions at random, and the cyclic patterns 72 are periodically arranged to facilitate the fabrication of the patterns on the uneven diffusing board 71 while suppressing the generation of moiré fringes.
FIGS. 27(a) and 27(b) are illustrations for explaining the function of the uneven diffusing board 71. The uneven diffusing board 71 has a multiplicity of concavities 73 disposed thereon at random. Since each of the concavities 73 is in the form of a concave lens, when light enters the same perpendicularly from the bottom side thereof as shown in FIG. 27(a), the incident light is diffused about the optical axis by the effect of the concave lens. Therefore, the uneven diffusing board 71 exhibits diffusion characteristics as shown in FIG. 27(b). When it is assumed that n represents the refractive index of the uneven diffusing board 71 and that ε represents the slope of a line segment connecting the center of a concavity 73 and an edge (apex) of the same. In the diffusion characteristics, there are peaks in positions expressed by φx=φy=±ε/(n−1) on both sides of a high peak in the middle. The diffusing characteristics are rotation-symmetric about the z-axis when the concavity 73 is circular.
FIG. 28 is a graph showing actual diffusing characteristics of the uneven diffusing board 71, in which diffusion characteristics in the φx direction and diffusion characteristics in the φy direction coincide with each other. When the uneven diffusion plate 71 having such characteristics is substituted for the uneven diffusing board 41 in the second example of the related art, the directional characteristics shown in FIG. 9 after transmission through the prism sheet 42 are converted as shown in FIG. 29 after transmission through the uneven diffusion plate 71. As a result, the directional characteristics are expanded at about 20° in both of the φx and φy directions, which provides a viewing angle that is sufficient when employed in a liquid crystal display.
No moiré fringe is generated in such uneven diffusing board 71 because the concavities 73 are arranged at random. However, since the concavities 73 are disposed at random, local polarization occurs in the disposition of them to produce differences in emitting intensity between the pixels of a liquid crystal display panel, which has resulted in the problem of color flickers just as in the uneven diffusing board 41. The problem of color flickers similarly occurs when used in the third example of the related art.
Since the uneven diffusing board 71 similarly diffuses light in all directions about the optical axis, a problem has arisen in that it reduces the luminance of the screen of a liquid crystal display by about 15% when compared to the uneven diffusing board 41 and the uneven diffusing board 61.
As described above, an uneven diffusing board formed with linear patterns has the problem of moiré fringes and color flickers, and an uneven diffusing board having granular patterns disposed thereon at random has the problem of color flickers and a reduction in luminance. The elimination of those problems is therefore demanded.
The examples of the related art described above are disclosed in Patent Document 1.
Patent Document 1: JP-A-2003-215584